DESCRIPTION (Adapted from applicant's description): The candidate and his group have developed an ultrasound measure of diaphragm function in an animal model. This new metric has been applied to mechanically ventilated pediatric patients and can discriminate between children with clinically normal and abnormal diaphragm function. The investigators propose to further develop, validate, and apply these ultrasound measures in children with respiratory failure. Once these noninvasive ultrasound measures are fully developed, new medications and/or ventilatory strategies may be trialed in order to improve diaphragm function and reverse the respiratory failure found in these children. The investigators anticipate that these ultrasound measures will influence and alter the management of respiratory failure in critical care medicine in the same way echocardiography has revolutionized the assessment of myocardial function in cardiology. The study aims to: 1) quantify diaphragm function in normal children and those with respiratory failure using our present ultrasound techniques; 2) develop additional ultrasound metrics; 3) validate these ultrasound measures with standard pulmonary function testing and clinical outcomes; and 4) evaluate the efficacy of new medications and/or ventilatory strategies aimed at reversing respiratory failure in children.